The invention relates to brush wear indicators for signalling to an operator of a dynamoelectric machine the occurrence of a predetermined degree of wear of the brushes on the machine, more particularly, it relates to brush wear indicators that utilize an electromechanical transducer connected between a signal means and a movable portion of a brush-biasing spring to actuate the signal means responsive to the occurrence of a predetermined degree of brush wear.
In those dynamoelectric machines that utilize brushes to conduct current from a rotor to fixed associated electrical circuits by forming a sliding electrical contact between brushes and either a plurality of slip rings or commutator segments, there always exists the need to periodically replace the brushes as they become worn down by the sliding relationship. In most such machines currently in use, no special brush wear indicating means are used in association with the brushes; instead, the extent of brush wear is periodically determined by visually examining the condition of the brushes. Of course, such physical examination is time consuming and often somewhat difficult to perform because it necessitates removal of housing covers in order to gain access to the brushes for inspection purposes. In addition, in such machines there always exists the danger that the rotor may be damaged if the brush is allowed to become so completely worn that the rivet attaching a shunt conductor to the brush scrapes the rotating commutator or slip ring. On the other hand, in many dynamoelectric machine applications, the anticipated brush life is sufficiently long to make it economically unfeasible to provide an additional brush wear indicating signal means. Also, in many applications of dynamoelectric machines, it is not imperative that the brushes be replaced before they are essentially completely worn out. In such applications a virtually completely worn out brush condition is relied on as a signal means for indicating that condition, and temporary loss of service due to the brush wear is not critical enough to economically justify the provision of separate brush wear indicating means on the machine.
Despite the fact that many dynamoelectric machines are satisfactorily operated without brush wear indicators, there also exists many applications in which brush wear indicators are desirable. In fact, prior to the invention disclosed herein, a number of different brush wear indicating mechanisms have been disclosed. For example, U.S. Pat. No. 2,691,114-Lykins, which issued on Oct. 5, 1954, shows a torsion-spring biased, relatively rigid brush follower arm that is effective to move a switch contact against a fixed switch contact responsive to a predetermined degree of brush wear, thereby to energize a signal circuit that indicates a given degree of such wear. Another type of brush wear indicator is shown in U.S. Pat. No. 3,523,288-Thompson, which issued on Aug. 7, 1970. The invention described in that patent includes a spring-biased pin that is mounted in sliding relationship on one side of a brush so that as the brush wears, the pin eventually drops into a recess in the brush thereby enabling a signal circuit switch to close with a snap action and alert an operator to the predetermined degree of brush wear indicated by the pin reaching the recess in the brush. A somewhat similar brush wear indicating means is shown in U.S. Pat. No. 4,121,207-Jones, which issued on Oct. 17, 1978 and discloses a microswitch used in combination with a switch actuating arm that rolls on the side of the brush until the roller reaches the outer end of the brush and drops off of it to close the micro-switch with a snap action. Yet another U.S. Pat. No. 4,172,988-Lowther, which issued Oct. 30, 1979, describes a brush wear indicator having an electrical contact mounted on the outer end of a brush so that the contact is moved into engagement with a fixed contact that is supported on an insulated member adjacent to the path of travel of the brush. In that system, when a brush is worn, it is moved toward a commutator or slip ring by a self-winding spring until the contact mounted on the brush engages the fixed contact thereby completing a circuit to an indicating signal that alerts an operator to a predetermined degree of brush wear.
All of the above-noted prior art brush wear mechanisms are either relatively expensive to manufacture because they require the assembly of a number of additional parts that normally are not required in brush holder and biasing structures, or they require the provision of extra movable switch contacts that are actuated by a mechanism that is directly mounted on a brush that is being monitored, as in the Lowther patent. In addition, some of these prior art mechanisms are relatively expensive because their use requires special machining and assembling procedures such as those needed to implement the invention described in the above-noted Jones patent wherein a separately mounted microswitch arm operates through an aperture that must be formed in a wall of a conventional brush holder.
In addition to the prior art brush wear indicators described by the above-mentioned patents, there is disclosed in a co-pending U.S. patent application Ser. No. 183,896 a brush wear indicator that utilizes a self-winding, brush-biasing spring of a relatively conventional brush holder assembly to control the energization of a brush wear indicating circuit. Briefly stated, in that invention, which is also assigned to the assignee of the present invention, a so-called negator spring of the brush holder assembly is positioned in electrical engagement with two or more electrical contacts to complete a circuit between the contacts when an associated brush in the brush holder is in a new, or relatively unworn, condition. As the brush wears, the negator spring winds on itself and sequentially moves out of contact with the respective fixed contacts, thereby de-energizing indicating circuits that are each connected, respectively, to one of the fixed contacts and a common contact. In another copending U.S. patent application, Ser. No. 183,920 there is disclosed another type of brush wear indicator that also utilizes a so-called negator spring of a relatively conventional type of brush holder assembly in order to control actuation of a brush wear indicating signal circuit. That invention is also assigned to the assignee of the present invention. In the apparatus disclosed by Michael, a negator spring is operable to close an indicating circuit to energize an indicating means, responsive to a coiled portion of the spring being moved into engagement with a flexible contact mounted near the fixed base of the spring, or into engagement with a switch mounted in a similar position, responsive to the occurrence of a predetermined degree of brush wear.
Although the inventions disclosed in those two copending patent applications utilize a self-winding negator spring to control actuation of a brush wear indicating signal means, they rely on a coiled portion of a negator spring to directly engage a circuit closing or actuating contact or switch rather than employing an electro-mechanical transducer connected between the negator spring and an indicating circuit to electrically isolate the spring from the circuit controlling means in the manner disclosed herein.
Accordingly, it is an object of the present invention to provide a brush wear indicator that is economical to manufacture, reliable in operation and overcomes the disadvantages of prior art brush wear indicators.
Another object of the invention is to provide a brush wear indicator including an electro-mechanical transducer that can be readily retrofitted to a conventional brush holder assembly to indicate a predetermined degree of brush wear responsive to a brush-biasing spring moving a preselected distance to thereby actuate the transducer and cause it to energize an indicating signal.
A further object of the invention is to provide a brush wear indicator that utilizes a relatively conventional self-winding brush-biasing spring of a brush holder assembly to operate an electro-mechanical transducer mounted on the brush holder radially outward from the path of movement of the brush-biasing spring, thereby to close a pair of relatively movable contacts which control actuation of a brush wear signal means.
Still another object of the invention is to provide a brush wear indicator in which a dielectric coupling is formed between a brush holder negator spring and a movable contact that is operated by a transducer to move into engagement with a fixed contact responsive to the occurrence of a predetermined degree of brush wear. An additional object of the invention is to provide a brush wear indicator that operates in combination with standard brushes, i.e., brushes that need not be modified from current commercially available designs in order to be compatible for use with the brush wear indicator.
Additional objects and advantages of the invention will become apparent to those skilled in the art from the description of it presented herein.